Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art and now comprise over 150 species of both naturally occurring and synthetic compositions. In general the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the framework crystal structure.
The early procedures for synthesizing zeolites produced materials that were very fine, often less than a few microns in size. Such small particle size powders are difficult to use in many industrial processes. Furthermore the small sizes can also create dust hazards for people who handle the materials. Larger zeolite particles, having average particle sizes of the order of 10 microns upwards, are preferred for many applications. However, for such large particle zeolites to be useful in all processes in which zeolites are used industrially, the large particle zeolites must retain the ion-exchange properties, adsorption capacity and selectivity, thermal stability and catalytic activity of finely divided crystalline zeolites. In addition, the large zeolite particles should exhibit high attrition resistance and crush strength.
Although larger zeolite bodies having sizes in excess of approximately five microns can be prepared by agglomerating small crystals, a typical agglomeration process requires a suitable binder, such as a clay, silica or alumina gel, or inorganic or organic adhesive, and processing conditions that assure reproducibility of the properties of the agglomerates. Because such processing conditions are often complex and difficult to control and because the binder material, which is a relatively inert material relatively incapable of adsorption, tends to reduce and/or modify the adsorptive and catalytic properties of the zeolite by dilution and in other ways, this approach is not optimal.
Accordingly, processes have been developed for producing zeolites having relatively large particle sizes ranging from tens of microns up to several millimeters or more. Many such processes begin with the preparation of a precursor, or preformed body, which contains certain reactive or unreactive kaolin-type clays and which can be converted by chemical means to a zeolite body that retains the shape of the preformed body.
Earlier preform-type products required expensive multiple synthesis steps, such as (1) admixing synthesized zeolite with kaolin clay and firing the composite to make it reactive, then further synthesizing additional zeolite from the reactive clay component by caustic treatment, or (2) bonding synthesized zeolite with silica sol, gelling, and further treating the composite with sodium aluminate at elevated pH to form additional zeolite (see German Patent No. 1,165,562 to Bayer). Cumbersome processes using sodium aluminate gel and water-immiscible hydrocarbon liquids have also been employed (see U.S. Pat. No. 3,094,383 assigned to Engelhard Corporation).
U.S. Pat. No. 2,992,068 describes a method of preparing zeolite A bodies from preformed bodies containing calcined kaolin clay, caustic and optionally added silica or alumina.
U.S. Pat. No. 3,065,054 describes a preparation of zeolite bodies from pre-formed bodies containing uncalcined kaolin clay, caustic and a porosity inducing agent.
U.S. Pat. No. 3,119,659 describes a method of preparing zeolite bodies from preformed bodies containing either calcined or uncalcined kaolin clay, or both, caustic and optionally added silica or alumina. The preformed bodies may also contain added zeolite powder.
U.S. Pat. No. 3,119,660 describes a method of preparing zeolite bodies from preformed bodies containing either calcined or uncalcined kaolin clay, or both, and optionally added silica or alumina. The preformed bodies may also contain included zeolite powder and diluents.
U.S. Pat. Nos. 3,367,886 and 3,367,887 describe methods of preparing zeolite bodies from preformed bodies containing zeolite powder, calcined and uncalcined kaolin clay and sodium hydroxide.
U.S. Pat. No. 3,370,917 describes a method of preparing Zeolite Z-12 bodies from preformed bodies containing calcined kaolin-type clay and sodium hydroxide in an Na.sub.2 O:SiO.sub.2 molar ratio of about 0.13.
U.S. Pat. No. 3,450,645 describes the preparation of zeolite bodies by extruding into pellets a mixture containing calcined and uncalcined kaolin-type clay, water and sodium hydroxide, aging and then ammonium-exchanging the pellets and finally digesting them in oil at 200.degree. F. (approximately 93.degree. C.).
U.S. Pat. Nos. 3,777,006 and 4,235,753 describe methods of preparing zeolite bodies from preformed bodies containing meta-kaolin and optionally caustic by treating those bodies with silicate solutions containing nucleation centers.
U.S. Pat. No. 3,909,076 describes the preparation of Zeolite X bodies from preformed bodies containing particles of both Zeolite X and meta-kaolin clay. The particles are stated to have an average size of from about 0.1 to about 50 microns, preferably from 0.5 to 10 microns.
U.S. Pat. Nos. 4,424,144 and 4,058,856 generally describe preparations of zeolite bodies from preformed bodies containing zeolite powder, meta-kaolin and sodium hydroxide.
However, the zeolite bodies prepared by these and similar methods frequently, and often unpredictably, exhibit poor crush strength and/or adsorption properties. In addition, the wet strength of the preformed bodies made by prior art methods is generally quite low; the preformed bodies that are converted to zeolite often disintegrate during aging and digestion, especially if any agitation is used. In addition, the rates of zeolite formation are generally slow; many of the examples in the prior art require three days or more reaction time.
There have recently been reported several classes of microporous compositions which are not zeolitic, and which will collectively be referred to hereinafter as "non-zeolitic molecular sieves", which term will be more precisely defined hereinafter. These non-zeolitic molecular sieves include the crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440 issued Jan. 12, 1982 to Wilson et al. These materials are formed from AlO.sub.2 and PO.sub.2 tetrahedra and have electrovalently neutral frameworks as in the case of silica polymorphs. Unlike the silica molecular sieve, silicalite, which is hydrophobic due to the absence of extra-structural cations, the aluminophosphate molecular sieves are moderately hydrophilic, apparently due to the difference in electronegativity between aluminum and phosphorus. Their intracrystalline pore volumes and pore diameters are comparable to those known for zeolites and silica molecular sieves.
In U.S. Pat. No. 4,440,871, there is described a novel class of silicon-substituted aluminophosphate non-zeolitic molecular sieves which are both microporous and crystalline. These materials have a three-dimensional crystal framework of PO.sub.2.sup.+, AlO.sub.2.sup.- and SiO.sub.2 tetrahedral units and, exclusive of any alkali metal or calcium which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular silicoaluminophosphate species involved; and "x", "y", and "z" represent the mole fractions of silicon, aluminum and phosphorus, respectively, present as tetrahedral oxides. The minimum value for each of "x", "y", and "z" is 0.01 and preferably 0.02. The maximum value for "x" is 0.98; for "y" is 0.60; and for "z" is 0.52. These silicoaluminophosphates exhibit several physical and chemical properties which are characteristic of both aluminosilicate zeolites and aluminophosphates.
In U.S. Pat. No. 4,500,651, there is described a novel class of titanium-containing non-zeolitic molecular sieves whose chemical composition in the as-synthesized and anhydrous form is represented by the unit empirical formula: EQU mR:(Ti.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Ti.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of between zero and about 5.0; and "x", "y" and "z" represent the mole fractions of titanium, aluminum and phosphorus, respectively, present as tetrahedral oxides.
In U.S. Pat. No. 4,567,029, there is described a novel class of crystalline metal aluminophosphate non-zeolitic molecular sieves having three-dimensional microporous framework structures of MO.sub.2, AlO.sub.2 and PO.sub.2 tetrahedral units and having an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(M.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (M.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; "M" represents at least one metal of the group magnesium, manganese, zinc and cobalt; and "x", "y", and "z" represent the mole fractions of the metal "M", aluminum and phosphorus, respectively, present as tetrahedral oxides.
In U.S. Pat. No. 4,544,143, there is described a novel class of crystalline ferroaluminophosphate non-zeolitic molecular sieves having a three-dimensional microporous framework structure of FeO.sub.2, AlO.sub.2 and PO.sub.2 tetrahedral units and having an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(Fe.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Fe.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; and "x", "y" and "z" represent the mole fractions of the iron, aluminum and phosphorus, respectively, present as tetrahedral oxides.
Other aluminophosphate and silicoaluminophosphate non-zeolitic molecular sieves are described in a number of pending patent applications, as described in more detail below.
The aforementioned patents and paten applications describe methods for the preparation of the non-zeolitic molecular sieves by hydrothermal crystallization thereof from a substantially homogeneous liquid reaction mixture containing reactive sources of aluminum, phosphorus, silicon (in the case of the silicoaluminophosphates) and the other element(s), if any, required in the non-zeolitic molecular sieve. The reaction mixture also preferably contains an organic templating, i.e., structure-directing, agent, preferably a compound of an element of Group VA of the Periodic Table, and/or optionally an alkali or other metal. The reaction mixture is generally placed in a sealed pressure vessel, preferably lined with an inert plastic material such as polytetrafluoroethylene and heated, preferably under autogenous pressure, at a temperature between 50.degree. C. and 250.degree. C., and preferably between 100.degree. C. and 200.degree. C., until crystals of the non-zeolitic molecular sieve product are obtained, usually for a period of from several hours to several weeks. Effective crystallization times of from about 2 hours to about 30 days are generally employed. The non-zeolitic molecular sieve is recovered by any convenient method such as centrifugation or filtration.
Although these hydrothermal crystallization methods are effective in producing the non-zeolitic molecular sieves in high yields, they have the disadvantage that, presumably because the crystallization of the non-zeolitic molecular sieve takes place from a substantially homogeneous and relatively high viscosity liquid or semi-gel, the average particle size of the non-zeolitic molecular sieve produced is often very small, typically in the sub-micron range. This problem of small particle size of the product is especially difficult in the SAPO molecular sieves of U.S. Pat. No. 4,440,871, and especially SAPO-34. Such small average particle sizes render the non-zeolitic molecular sieves difficult to filter or centrifuge, and hence difficult to separate cleanly from the reaction mixture in which they are formed. Moreover, the small average particle size of the non-zeolitic molecular sieve tends to cause the same problems (i.e., difficulty of use in some industrial applications, dust hazards, difficulty in binding without reduction and/or modification of adsorptive and catalytic properties, and lack of reproducibility in properties after binding) as with the small average particle size zeolites discussed above.
There is thus a need for a process for the production of non-zeolitic molecular sieves which will allow the non-zeolitic molecular sieves to be produced in a form having an average particle size substantially greater than that of the non-zeolitic molecular sieves produced by the hydrothermal crystallization processes described above, and this invention provides such a process.